In recent years, separation membranes have been utilized in various fields, such as a field of water treatment including water-purification treatment and effluent treatment, a field of medical uses including blood purification, a field of food industry and a field of using them as separators for batteries, charged membranes, electrolyte membranes for fuel cells and the like.
Of those fields, the field of drinking water production and the field of industrial water production in particular, namely the fields of water treatment for uses in water purification, effluent disposal and seawater desalination, have come to use separation membranes as alternatives to traditional sand filtration, coagulative sedimentation or evaporation, or for the purpose of enhancing the quality of treated water. Since the amount of treated water is great in such fields, as long as separation membranes deliver excellent water-permeation performance, it becomes possible to reduce the area of each separation membrane, whereby the apparatus can be made compact in size to result in saving on the cost of facilities, and the excellent water-permeation performance of the separation membranes also becomes advantageous in terms of membrane exchange cost and footprint of the apparatus.
As separation membranes for water treatment are used those appropriate to sizes of substances targeted for separation which are present in water to be treated. Since natural water usually contains lots of suspended solids, microfiltration membranes or ultrafiltration membranes for removal of suspended solids in water have generally been used. Depending on the water to be treated, there may be cases where water is contaminated with harmful metal ions, and these metal ions are too small in size to remove by the use of microfiltration membranes or ultrafiltration membranes. As a result, there has been a necessity to provide a step of removing metal ions in water in addition to the clarification step for removing suspended solids from the water.
On the other hand, it is known that the removal of metal ions in water have been carried out by adsorptive removal using an ion exchange resin, adsorptive removal using a chelating resin, and adsorptive removal using an inorganic adsorbent such as a cerium compound (Patent Document 1). However, these adsorptive removals have not only economic problems concerning the cost of facilities including an adsorption tower, initial investments in resins, the cost of resin regeneration and so on, but also a practical utility problem, such as clogging of a water channel due to adsorption of suspended solids in water onto adsorbents. For this reason, there has been a need to provide a step for removing suspended solids in water in addition to the adsorption step for removing metal ions.
Further, there have been known the membrane provided with an adsorption layer having chelating functional groups through graft polymerization only at the surface or in a surface layer thereof (Patent Document 2) and the membrane prepared by introducing glycidyl methacrylate onto the surface of a porous membrane and the inner surfaces of pores in the porous membrane through the graft polymerization and then chemically introducing thereto chelating functional groups (Patent Document 3). In those cases, places where chemicals are apt to reach, e.g. surfaces of large pores, are given priority in introducing the chelating groups, while the chelating groups are hard to be introduced onto places where chemicals are hard to reach, e.g. the interior of fine pores. Therefore, those membranes had problems that the distribution of chelating functional groups was nonuniform and it was difficult to secure sufficient capability to adsorb metal ions, and further problems of adsorption and clogging of water channel due to suspended solids in water. The cloth substrate provided with chelating functional groups (Patent Document 4) and the chelating functional group-containing fiber (Patent Document 5) have also been known, but they each had practical utility problems, such as adsorption and clogging of water channel due to suspended solids in water, and hence there was a necessity to provide the step for removing suspended solids in water in addition to the adsorption step for removing metal ions.
Under these circumstances, a composite separation membrane for performing removal by clarification and removal by adsorption at the same time has been disclosed (Patent Document 6). Therein, it is disclosed that filtration through the composite separation membrane provided with a layer having a three-dimensional network structure and a layer having a porous structure and containing an adsorbent allowed removal of suspended matter and metal ions (including boron ions) from seawater.